Hydrocarbon oil compositions typically comprise a mixture of at least one hydrocarbon base oil and one or more additives, where each additive is employed for the purpose of improving the performance and properties of the base oil in its intended application; e.g., as a lubricating oil, heating oil, diesel oil, middle distillate fuel oil, and so forth. Lubricating oil compositions face rather stringent viscosity requirements, as set, for example, by SAE specifications. Such compositions must meet a minimum viscosity requirement at high temperature (at least about 100.degree. C.) and a maximum viscosity requirement at low temperature (about -5.degree. to -30.degree. C.). The minimum viscosity requirement at high temperature is intended to prevent the oil from thinning during engine operation to the point at which excessive engine wear and increased oil consumption would result. The maximum viscosity requirement at low temperature facilitates engine start-up in cold weather and also ensures the cold oil has sufficient pumpability and flowability to avoid engine damage due to insufficient lubrication. Simple blends of base oils having different viscosity characteristics generally do not meet the low and high temperature viscosity requirements of multigrade lubricating oils. The primary tool for meeting the requirements is the use of viscosity modifiers, also referred to as viscosity index improvers or V.I. improvers.
A viscosity modifier is conventionally an oil-soluble polymer, often a hydrocarbon-based polymer with a number average molecular weight in the range of about 20,000 to 200,000. These polymers, with their relatively large size, can significantly increase kinematic viscosities of base oils even at low concentrations. However, lubricating oil solutions containing these viscosity modifiers are non-Newtonian in nature. Consequently, these solutions tend to give lower viscosities than expected in a high shear environment. This behavior is believed to arise from shear-induced alignment of the V.I. polymer chains. In an operating internal combustion engine, viscosity modifiers therefore increase the low temperature, high shear (CCS) viscosity of a lubricating base oil to a lesser extent than they increase its high temperature viscosity. A point can be reached where the amount of viscosity modifier added to achieve the required minimum viscosity of the multigrade oil at high temperature precludes the possibility of meeting the required maximum viscosity at low temperature. Consequently, constraints exist on the amount of viscosity modifier which can be employed to meet the low and high temperature viscosity requirements of a lubricating oil composition.
Lubricating oil compositions generally also contain polymeric dispersant additives, whose primary function is to maintain insolubles formed by oxidation, etc. in a suspension in the oil and thus avoid sludge flocculation and precipitation. The dispersants are typically hydrocarbon polymers having a number average molecular weight ("M.sub.n ") of about 10,000 or less that have been chemically modified to contain polar groups; e.g., nitrogen- and ester-containing groups. The amount of dispersant employed is dictated and controlled by the effectiveness of the particular material in achieving its dispersant function. Engine oils commercially available at U.S. service stations typically contain about two to four times as much dispersant as viscosity modifier, based on active ingredient.
Conventional dispersants can increase the low and high temperature viscosity characteristics of a base oil by virtue of their polymeric nature. However, because dispersant polymer molecules are much smaller than those of a viscosity modifier, the dispersant is much less shear sensitive. As a result, a dispersant contributes more to the low temperature viscosity of the lubricating oil relative to its contribution to the high temperature viscosity than does a viscosity modifier. Moreover, the dispersant, with its lower degree of polymerization, contributes much less to the high temperature viscosity of the base oil in an absolute sense than does the viscosity modifier. Thus, the magnitude of the low temperature viscosity increase induced by the dispersant can exceed the low temperature viscosity increase induced by the viscosity modifier without the benefit of a proportionately greater increase in high temperature viscosity as obtained from a viscosity modifier. Consequently, as the dispersant-induced low temperature viscosity increase causes the low temperature viscosity of the oil to approach the maximum low temperature viscosity permitted, it becomes increasingly difficult to introduce an amount of viscosity modifier sufficient to meet the minimum viscosity required at high temperature without crossing the low temperature viscosity threshold.
A continuing need exists for the development of improved dispersants which can provide viscosity benefits to lubricating oil compositions and at the same time achieve a dispersant function equal to or surpassing that of conventional dispersants.
Various oil-soluble, hydrocarbon-polymer-based dispersant additives suitable for use in lubricating oil compositions are disclosed in the art. EP-A-208560, for example, discloses dispersant additives formed by reaction of an amine, alcohol, amino alcohol and mixtures thereof with a polyolefin-substituted dicarboxylic acid material. The polyolefin-substituted dicarboxylic acid material is prepared by reaction of a polyolefin (e.g., polyisobutylene) having M.sub.n of 1500 to 5,000 with an unsaturated dicarboxylic acid reactant (maleic anhydride), such that the polymer product (polyisobutenyl succinic anhydride) contains 1.05 to 1.25 moles of dicarboxylic acid producing moieties per mole of polyolefin used in the reaction. Suitable polyolefins include homopolymers and copolymers of C.sub.2 -C.sub.10 monoolefins.
U.S. Pat. No. 5,229,022 teaches lubricating additives comprising ethylene-.alpha.-olefin copolymers terminally substituted with mono- or dicarboxylic acid producing moieties. The acid-substituted polymer can be further reacted with nucleophilic reagents such as amines, alcohols, and metal compounds to give other materials useful as additives, such as dispersants.
EP-A-490454 teaches alkenyl succinimide derivatives useful as dispersant additives, wherein the alkenyl group is derived from a terminally unsaturated atactic propylene oligomer. The succinimide derivative is formed by reacting the succinated oligomer with a C.sub.1 -C.sub.50 amine.
Polyisobutylene ("PIB") has often been the polymer of choice in polymeric dispersants, chiefly because it is readily available by cationic polymerization from butene streams. PIB is a substantially amorphous material which generally contains residual unsaturation amounting to about one ethylenic double bond positioned along each polymer chain. The double bonds serve as the sites for functionalizing PIB by reaction with, for example, unsaturated carboxylic compounds such as maleic anhydride.
As indicated above, ethylene-.alpha.-olefin copolymers and .alpha.-olefin homo- and copolymers have also been disclosed to be useful in polymeric dispersants. The polymers prepared by polymerization of the corresponding monomers using conventional Ziegler-Natta catalysts are generally not suitable for use as backbones for lubricating oil dispersant additives. Conventional Ziegler-Natta catalysts have relatively high activity and several types of active sites, resulting in .alpha.-olefin polymers which have M.sub.n 's above 10,000 and relatively broad molecular weight distributions ("MWD"). Low molecular weight polymers can be obtained with these catalysts using a molecular weight regulator such as hydrogen, but this saturates the double bonds in the polymers, destroying the polymer's reactivity in functionalization chemistries useful for producing dispersants which rely on a high double bond content; e.g., functionalization with unsaturated carboxylic compounds such as maleic anhydride.
Ethylene-.alpha.-olefin copolymers and .alpha.-olefin homo- and copolymers in the dispersant molecular weight range can be prepared by polymerization of the corresponding monomers using catalysts composed of a metallocene compound (i.e., a cyclopentadienyl-containing transition metal compound) and a suitable activator or cocatalyst such as aluminoxane. For example, U.S. Pat. No. 5,229,022, noted above, describes the use of metallocenes for the preparation of ethylene-.alpha.-olefin copolymers. JP-A-63/057615 discloses liquid .alpha.-olefin random copolymers prepared by copolymerization of C.sub.3 to C.sub.20 .alpha.-olefins in the presence of catalysts formed from cyclopentadienyl zirconium hydride compounds and aluminoxane. The copolymers have 1-99 mole % of units derived from one of the monomers present in the copolymer, an inherent viscosity of 0.005-4 dl/g, MWD of no more than 3, iodine number of 0 to 85, and a C13-NMR spectrum indicating the copolymer has a regular head to tail structure. The copolymer is disclosed to contain terminal unsaturation on one end of the copolymer chains. Example 4 describes the preparation of a 1-butene-1-hexene copolymer with 15 mole % butene content, intrinsic viscosity of 0.02 dl/g, MWD of/1.82, and iodine number of 51.
JP-A-63/037102 discloses a modified liquid .alpha.-olefin polymer consisting of an unsaturated C.sub.3 to C.sub.10 carboxylic acid, anhydride, or ester thereof, bonded to an .alpha.-olefin homo- or copolymer prepared by polymerizing one or more C.sub.3 to C.sub.20 .alpha.-olefins using a zirconocene-aluminoxane catalyst system. The modified polymer is useful, inter alia, as an additive for lubricating oils. The starting polymer is disclosed to have an intrinsic viscosity of 0.005 to 0.4 dl/g, M.sub.n of 300 to 8,000, MWD of 3 or less, and a C-13 NMR spectrum indicative of head-to-tail bonding, and an isotactic mm triad fraction of 0.35 or less. Example 4 discloses an n-butyl methacrylate-modified liquid propylene-butene-1 copolymer, prepared by heating the liquid polymer (which has 76 mole % propylene content, intrinsic viscosity of 0.03 dl/g, Mn of 730, MWD of 1.88, and an iodine value of 35) together with n-butyl methacrylate.
JP-A-01/132605 discloses epoxidated liquid .alpha.-olefin polymers which can be prepared from a liquid .alpha.-olefin polymer substantially like that disclosed in JP-A-63/037102 discussed in the last paragraph. The epoxidated polymers are useful, inter alia, as compatibility agents in lubricating oil compositions.
DE-A-4030399 teaches polymers and oligomers of propylene having functional end groups, which are produced by reaction of hetero-atom-containing organic compounds with polymers and oligomers composed of propylene and 0-40 wt. % of another C.sub.2 to C.sub.8 1-alkene. The polymers and oligomers have M.sub.n of 100 to 100,000, MWD of 1 to 3, and one unsaturated chain end. They are preferably prepared by polymerization of the monomer(s) in the presence of a bridged metallocene-aluminoxane catalyst system. Example 1 discloses the preparation of an isotactic propylene oligomer (M.sub.n =800;MWD=2.3) using a silane-bridged zirconocene. Subsequent examples disclose the preparation of anhydride-, mercaptan-, epoxide-, and carboxylic acid- terminated oligomers, among others, from the propylene oligomer prepared in Example 1. The hetero-atom functionalized polymers and oligomers are compatible with, and can be combined with, polar polymers to give alloys or copolymers.